Fix #2607: handle frozen dataclass exceptions

[sb123sb123]

Fixes #2607

When strict_exception_groups=False collapses a single exception group, Trio updates traceback and context metadata on the contained exception. Frozen dataclass exceptions reject normal attribute assignment, so the original exception is replaced by FrozenInstanceError.

This change uses object.__setattr__ for Trio internal traceback/context restoration paths and adds a regression test through a real nursery.

Testing:

• PYTHONPATH=src .venv/Scripts/python.exe -m pytest src/trio/_core/_tests/test_run.py -q - 155 passed, 1 skipped
• PYTHONPATH=src .venv/Scripts/python.exe -m pytest src/trio/_tests/test_util.py -q - 8 passed
• .venv/Scripts/ruff.exe check src/trio/_core/_run.py src/trio/_core/_tests/test_run.py src/trio/_util.py - passed
• python -m black --check src/trio/_core/_run.py src/trio/_core/_tests/test_run.py src/trio/_util.py - passed
• Direct Windows reproduction with a frozen dataclass exception - returned the original FrozenError

A baseline run of src/trio/_core/_tests/test_io.py had 28 passed and 1 failure because this Windows host emits localized text for WinError 10038 while the existing test regex only accepts the English message; this is unrelated to the change.

AI assistance was used to investigate, implement, and test this patch. The reported results are from commands actually run on the Windows host.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (0480602) to head (4547a15).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3522   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19471        19483   +12     
  Branches         1323         1323           
===============================================
+ Hits            19471        19483   +12     

Files with missing lines	Coverage Δ
src/trio/_core/_run.py	100.00000% <100.00000%> (ø)
src/trio/_core/_tests/test_run.py	100.00000% <100.00000%> (ø)
src/trio/_util.py	100.00000% <100.00000%> (ø)

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[A5rocks]

Thanks, but I don't want to fix this. strict_exception_groups=False is deprecated and is due to be removed in a couple months. Besides, this is a behavior change. Maybe some exceptions rely on their custom __setattr__ in ways other than being frozen!